Previous work by this investigator has been directed toward the isolation and purification, to near homogeneity, of 200,000 dalton core-particles of nuclear-protein subunits obtained from nuclease digestion of chromatin. Further study of the detailed molecular structure of these core-particles will be carried out. The subunit structure and microheterogeneity of chromatin core-particles from chicken erythrocytes and developing sea urchin embryos will be examined by purifying to homogeneity different subfractions of the nucleoprotein complexes. The principal aims are to: (1) Isolate and obtain three-dimensional structural analysis on a homogeneous core-particle fraction. (2) Obtain knowledge of the spatial arrangement of "inner" histone proteins and DNA in the core-particles, in order to understand the function of the subunit structure in chromatin. (3) To relate this structure to problems of gene expression and DNA replication.